OFDM communication channel

ABSTRACT

In an OFDM-based receiver, means for achieving time synchronization comprising: A. means for extracting pilot signals contained in the OFDM received signal; B. means for analyzing the pilot signals in the frequency domain and for issuing a signal indicative of a synchronization error in the received signal; C. means for correcting the synchronization error responsive to the signal indicative of the synchronization error. In an OFDM-based receiver, automatic frequency correction means in a subscriber unit comprising: A. an inner frequency correction loop for generating a LO frequency related to a frequency of a received signal; B. an outer frequency correction loop for correcting the LO frequency according to instructions received from a base station. In an OFDM-based receiver, a channel sounder comprising: A. means for extracting pilot signals contained in the OFDM received signal; B. means for analyzing the pilot signals in the frequency domain and for issuing signals indicative of a distortion in each pilot signal, wherein each of said pilot distortion signals comprises both an amplitude and a phase component; C. means for analyzing the signals indicative of a distortion in each pilot signal and for computing therefrom corrective signals for correcting distortions in the received signal.

The present application is a divisional from U.S. patent applicationSer. No. 09/493,662 entitled “OFDM communication channel” filed on 28Jan. 2000, and claims priority therefrom.

FIELD OF THE INVENTION

This invention relates to OFDM communication channels, and moreparticularly to improvements in channel performance using signalprocessing of pilot signals in the channel.

BACKGROUND OF THE INVENTION

Advanced communications today may use the Orthogonal Frequency DivisionMultiplex (OFDM) modulation for efficient transmission of digitalsignals. These signals may include video, voice and/or data. OFDM is acommonly used implementation of Multi-Carrier Modulation (MCM).

The Orthogonal Frequency Division Multiplex (OFDM) is a modern advancedmodulation method, that achieves better use of the frequency spectrum.

OFDM has been used in recent years in many applications where robustnessagainst severe multipath and interference conditions is required, or ahigh system capacity, flexibility in providing variable bit rateservices, scalability and a capability to perform well in SingleFrequency Networks (SNF). OFDM forms the basis for various communicationstandards, including for example the Digital Terrestrial TelevisionBroadcasting, wireless LANs and Wireless Local Loops.

OFDM requires an advanced signal processing.

Thus, a block of information is divided among N frequency channels, sothat a portion of the information is transmitted in each of theabovementioned channels or frequencies. Since each channel is orthogonalto the others, a better utilization of the frequency spectrum isachieved.

In OFDM, since each symbol is N times longer, the percent overlapbetween adjacent symbols decreases, hence the Inter-Symbol InterferenceISI is lower. Still better spectrum utilization is achieved by QAM(Quadrature Amplitude Modulation) on each of the N carriers.

An IFFT (Inverse Fourier Transform) is performed on the modulatedcarriers, to form the signal in the time domain that corresponds to theabove modulated carriers. The signal is transmitted as a frame thatcontains the block of information to be transmitted.

A possible problem in the above modulation scheme may be an error in thetime synchronization between signals.

When there is a time synchronization error, the signals after FFT in thevarious subchannels are rotated with respect to each other.

This effect creates interference within the subchannel.

Another problem is a frequency error between the transmitted signal andthe receiver. A frequency error generates a frequency shift that maychange the location of symbols and/or may generate interference betweensymbols.

Because of channel imperfection, a time or phase delay may be generatedbetween the various parts of the spectrum of the transmitted signal.This distortion of the frequency spectrum of the transmitted signal mayinterfere with the signal reconstruction in the receiver.

The problem is further aggravated by multipath.

Multipath may cause several replicas of a signal to be received, eachpossibly having a different time delay, amplitude and polarity.

These signals may result in interference between adjacent transmittedframes.

Prior art systems apparently are different.

Thus, Seki et al., U.S. Pat. No. 5,771,224 , discloses an orthogonalfrequency division multiplexing transmission system and transmitter andreceiver therefor. It transmits an OFDM transmission frame, with nullsymbols and reference symbols being placed in the beginning portion ofthe frame and QPSK symbols are placed in an information symbol dataregion in the frame, with equal spacing in time and frequency.

The carrier amplitude and phase errors are corrected by a correctioninformation producing section on the amplitude and phase variations ofthe received signal detected by the variation detector to producecorrected information.

Baum et al., U.S. Pat. No. 5,802,044 , discloses a multicarrier reverselink timing synchronization system. A center station transmits a forwardlink signal, receives a reverse link signal, and determines a timingoffset for signals received on a reverse link timing synchronizationchannel.

A reverse link symbol timing synchronization can be used in a systemhaving a plurality of transmitting overlap bandwidth subscriber units onan OFDM-like spectrally overlapping reverse channel. The modulationmethod may comprise M-ary Quadrature Phase Shift Keying(M-PSK), M-aryQuadrature Amplitude Modulation (QAM) or other digital modulationmethod.

Gudmundson et al., U.S. Pat. No. 5,790,516 , discloses a method andsystem for pulse shaping for data transmission in an orthogonalfrequency division multiplexed (OFDM) system.

Yamauchi et al., U.S. Pat. No. 5,761,190, discloses an OFDM broadcastwave receiver. An OFDM (Orthogonal Frequency Division Multiplex)broadcast wave receiver for receiving an OFDM broadcast wave.

It automatically discriminates whether the received signal is of a wideband or a narrow band by determining if a carrier signal having apredetermined frequency is present among signals of a plurality offrequencies, acquired by OFDM demodulation of the reception signal bydemodulation means.

It also controls the demodulating operation of the demodulation means inaccordance with the discrimination result to thereby acquire ademodulated signal.

Schmidl et al., U.S. Pat. No. 5,732,113, discloses a a method for timingand frequency synchronization of OFDM signals. It relates to a methodand apparatus that achieves rapid timing synchronization, carrierfrequency synchronization, and sampling rate synchronization of areceiver to an orthogonal frequency division multiplexed (OFDM) signal.The method uses two OFDM training symbols to obtain full synchronizationin less than two data frames. A first OFDM training symbol has onlyeven-numbered sub-carriers.

A second OFDM training symbol has even-numbered sub-carriersdifferentially modulated relative to those of the first OFDM trainingsymbol by a predetermined sequence.

Synchronization is achieved by computing metrics which utilize theunique properties of these two OFDM training symbols. Timingsynchronization is determined by computing a timing metric whichrecognizes the half-symbol symmetry of the first OFDM training symbol.Carrier frequency offset estimation is performed in using the timingmetric as well as a carrier frequency offset metric which peaks at thecorrect value of carrier frequency offset. Sampling rate offsetestimation is performed by evaluating the slope of the locus of pointsof phase rotation due to sampling rate offset as a function ofsub-carrier frequency number.

Awater et al., U.S. Pat. No. 5,862,182, discloses an OFDM digitalcommunications system using complementary codes.

The encoding/transmission of information in an OFDM system is enhancedby using complementary codes. The complementary codes, moreparticularly, are converted into phase vectors and the resulting phasevectors are then used to modulate respective carrier signals. Themodulated result is then transmitted to a receiver which decodes thereceived signals to recover the encoded information.

Isaksson, et al. U.S. Pat. No. 5,812,523, discloses a method and devicefor synchronization at OFDM-system.

A method of demultiplexing OFDM signals and a receiver for such signals.

The method is concerned with synchronization in an OFDM receiver. Asignal is read into a synchronization unit, in the time domain, i.e.,before Fourier transforming the signal by means of an FFT processor. Inthe synchronization unit, a frame clock is derived for triggering thestart of the FFT process and for controlling the rate at which data issupplied to the FFT processor. For OFDM reception, it is vital that theFFT process commences at the right point in time. Once the frame clockhas been recovered, a frequency error can be estimated by thesynchronization unit. The frequency error is used to control anoscillator which generates a complex rotating vector which is, in turn,multiplied with the signal to compensate for frequency errors. Themethod can be used both with OFDM systems in which symbols are separatedby guard spaces, and with OFDM systems in which symbols are pulseshaped.

Kim, U.S. Pat. No. 5,963,592, discloses an adaptive channel equalizerfor use in digital communication system utilizing OFDM method. Anadaptive channel equalizer for use in OFDM receiver is disclosed. Theadaptive channel equalizer comprises a first complex multiplier foroutputting a first in-phase complex multiplication signal and a firstquadrature phase complex multiplication signal; a reference signalgenerator for generating a reference signal; an error calculator foroutputting an in-phase error signal and a quadrature phase error signal;a delay unit for outputting an in-phase delay signal and a quadraturephase delay signal; a gain controller for outputting an in-phase gaincontrol signal and a quadrature phase gain control signal;

-   -   a second complex multiplier for outputting a second in-phase        complex multiplication signal and a second quadrature phase        complex multiplication signal; an adder for outputting updated        in-phase and quadrature phase coefficients; an address generator        for generating a write address signal and a read address signal;    -   a storage unit for storing the updated in-phase and quadrature        phase coefficients, and outputting the updated coefficients; an        initial coefficients generator for generating an initial        coefficients; a selecting signal generator for generating a        selecting signal; and a multiplexing unit for selecting one of        the initial coefficients and the updated coefficients according        to the selecting signal.

Seki et al., U.S. Pat. No. 5,694,389, discloses an OFDMtransmission/reception system and transmitting/receiving apparatus. Theapparatus improves the frequency acquisition range and the resistance tomultipath interference. In a digital signal transmission system usingOFDM, on the transmission side, some or all of a plurality ofequidistant carrier positions are treated as reference carrierpositions. The actual transmitted carriers are arranged in apredetermined pattern non-equidistant to the frequency carrier positionsto form an OFDM symbol.

This OFDM symbol is periodically transmitted as frequency referencesymbols. On the reception side, the carrier arrangement pattern of thefrequency reference symbols is detected, a carrier frequency offset isdetected from the detected pattern offset, and the carrier frequency iscompensated based on the frequency offset.

Cimini et al., U.S. Pat. No. 5,914,933, discloses a clustered OFDMcommunication system. A multicarrier communication system for wirelesstransmission of blocks of data having a plurality of digital datasymbols in each block. The communication system includes a device fordistributing the digital data symbols in each block over a plurality ofclusters, each of the clusters receiving one or more digital datasymbols. The digital data symbols are encoded in each of the cluster;and modulated in each cluster to produce a signal capable of beingtransmitted over the sub-channels associated with each cluster.

A transmitter thereafter transmits the modulated signal over thesub-channels. By distributing the modulated signal over a plurality ofclusters, overall peak-to-average power (PAP) ratio is reduced duringtransmission and transmitter diversity is improved.

Williams et al., U.S. Pat. No. 5,815,488, discloses a multiple useraccess method using OFDM. A communication method enables a plurality ofremote locations to transmit data to a central location. The remotelocations simultaneously share a channel and there is a high degree ofimmunity to channel impairments.

At each remote location, data to be transmitted is coded by translatingeach group of one or more bits of the data into a transform coefficientassociated with a frequency in a particular subset of orthonormalbaseband frequencies allocated to each remote location. The particularsubset of orthonormal baseband frequencies allocated to each location ischosen from a set of orthonormal baseband frequencies. At each remotelocation, an electronic processor performs an inverse orthogonaltransform (e.g., an inverse Fourier Transform) on the transformcoefficients to obtain a block of time domain data. The time domain datais then modulated on a carrier for transmission to the central location.

Preferably, the time intervals for data transmission at the differentremote locations are aligned with each other. In one embodiment of theinvention, all of the baseband frequencies are allocated to a singleparticular remote location for one time slot. At the remote location,data is received from a plurality of remote locations. The data isdemodulated to obtain baseband time domain data. The orthogonaltransform is performed on this data to obtain transform coefficients.Each transform coefficient is associated with a baseband frequency. Thecentral location keeps track of which baseband frequencies are allocatedto which remote location for subsequent translation of each transformcoefficient.

Isaksson, U.S. Pat. No. 5,726,973, discloses a method and arrangementfor synchronization in OFDM modulation. A method and an arrangement forsynchronization in OFDM modulation. Frequency errors of an IF clock anda sampling clock are controlled by estimating the deviation of thesampling clock and, respectively, the IF clock for two subcarriers withdifferent frequencies.

According to the invention, the frequencies are chosen symmetricallyaround zero and the absolute phase errors are detected for bothsubcarriers.

Timing errors and phase errors are formed from the absolute phase errorsin order to generate two control signals. The first control signal isformed from the deviation of the sampling clock and the timing error forcontrolling the sampling clock while the second control signal is formedfrom the deviation of the IF clock and the phase error for controllingthe IF clock.

Wright, U.S. Pat. No. 5,838,734, discloses means for compensation forlocal oscillator errors in an OFDM receiver. A receiver for orthogonalfrequency division multiplexed signals includes means for calculatingthe (discrete) Fourier Transform of the received signal, and means forcalculating the phase error due to local oscillator errors.

McGibney, U.S. Pat. No. 5,889,759, discloses an OFDM timing andfrequency recovery system. A synchronizing apparatus for a differentialOFDM receiver that simultaneously adjust the radio frequency and sampleclock frequency using a voltage controlled crystal oscillator togenerate a common reference frequency. Timing errors are found byconstellation rotation. Subcarrier signals are weighted by using complexmultiplication to find the phase differentials and then the timingerrors. The reference oscillator is adjusted using the timing errors.Slow frequency drift may be compensated using an integral of the timingerror. Frequency offset is found using the time required for the timingoffset to drift from one value to another.

Background material on advanced modulation techniques and relatedcommunication topics may be found in the following articles:

Scott L. Miller and Robert j. O'Dea, “Peak Power and Bandwidth EfficientLinear Modulation”, IEEE transactions on communications, Vol. 46, No.12, pp. 1639-1648, December 1998.

Kazuki Maeda and Kuniaki Utsumi, “Bit-Error of M-QAM Signal and itsAnalysis Model for Composite Distortions in AM/QAM Hybrid Transmission”,IEEE transactions on communications, Vol. 47, No. 8, pp. 1173-1180,August 1999.

Kazuki Maeda and Kuniaki Utsumi, “Performance of Reduced-Bandwidth 16QAM with Decision-Feedback Equalization”, IEEE transactions oncommunications, Vol. COM-35, No. 7, pp. 1173-1180, July 1987.

Background material on phase noise in advanced communication systems maybe found in the following references:

Yossi Segal and Zion Hadad, “OFDMA access method for HIPERACESS”,HARNCl.doc, December 1999.

Naftali Chayat, “Updated Submission Template for TGa—Revision 2”, IEEE802.11-98/156r2, March 1998.

Alcatel, Bosch, Ericsson, Lucent, Nokia, Siemens AG and Siemens ICN,“Proposal for the Adoption of the TDMA Access Scheme in HIPERACCESS”,HA16ERI1a.doc, December 1999.

Thierry Pollet, Mark Van Bladel and Marc Moeneclaey, “BER Sensitivity ofOFDM Systems to Carrier Frequency Offset and Wiener Phase Noise”, IEEEtransactions on communications, Vol. 43, No. 2/3/4, pp. 191-193,February/March/April 1995.

Luciano Tomba, “On the Effect of Wiener Phase Noise in OFDM Systems”,IEEE transactions on communications, Vol. 46, No. 5, pp. 580-583, May1998.

Naftali Chayat, “TGa Comparison Matrix per 98/156r2”, IEEE802.11-98/157r5, May 1998.

ETSI EP BRAN #16 Athens, Greece November 29- Dec. 3, 1999HA16RNC1Annex.doc page 3 of 13 22-Nov.-99

SUMMARY OF THE INVENTION

The present disclosure relates to improvements in OFDM-based digitalcommunications. The scope and spirit of the invention are betterdescribed with the inclusion of specific applications thereof.

A possible problem in the above modulation scheme may be an error in thetime synchronization between several signals appearing at the receiver,or between transmitter and receiver.

When there is a time synchronization error, the signals after FFT in thevarious subchannels are rotated with respect to each other.

This effect creates interference within the subchannel.

One application of the invention relates to receiver synchronizationusing means for Automatic Synchronization Control (ASC).

The ASC means use an analysis of pilot signals in the transmitted signalto implement the ASC loop.

The analysis is performed continuously, in real time. The correction ofdetected errors is also performed continuously in real time.

The time synchronization error may be evaluated based on the rate ofrotation of the pilot signals. A correction signal is generatedaccordingly, to adjust the timing in the receiver to the receivedsignal. This is implemented in an ASC loop, to achieve optimal timingfor sampling in the A/D converter.

Another problem is a frequency error between the transmitted signal andthe receiver.

A frequency error generates a frequency shift that may change thelocation of symbols and/or may generate interference between symbols.The information may be divided between separate bins, or may be assignedto other than the desired bins. Some information may be lost because ofthe frequency shift. The actual effect in each case (or at any instantin time) depends on the measure of frequency deviation.

Real-time means are used to measure the frequency error and correct forit in an Automatic Frequency Control (AFC) loop.

A correction signal is generated accordingly, to correctly tune thereceiver to the received signal.

Thus, the system will adapt to varying channel characteristics in realtime, to achieve improved communications.

This may be useful in DVB-T, for example, where there are a large numberof pilot signals available.

The frequency resulting from the AFC loop is used as a clock for thereceiver and subsequently for the transmitter. A frequency error maystem from two possible causes:

-   -   A. an undesired difference between the receiver LO (local        oscillator) and the transmit LO.    -   B. a frequency Doppler shift because of the movement of the        mobile subscriber.

This effect, together with means for its correction using a dual loopAFC, are detailed elsewhere in the present disclosure.

A second application relates to a channel sounder. Using means foranalyzing the received pilot signals, a signal processor cancharacterize the communication channel. Using the pilots rather than theinformation or noise in the channel may achieve a better performancesystem.

The phase and amplitude of the pilots is measured to evaluate thechannel distortion at different frequencies. The results are used toapply a correction to the received signal whose subcarriers are locatedbetween the pilot signals.

In one embodiment, the average distortion of two adjacent pilots is usedto correct the information between these pilots. When the distortion ineach pilot is different, the correction may be in error.

A better correction may be achieved using an interpolation process tocorrect for phase and amplitude of received signals between any twoadjacent pilots. This corrects the distortion of the signal frequencyspectrum, to improve the receiver performance.

Interpolation may be used to arrive at a channel estimate for eachchannel frequency, and to correct the signal accordingly. The correctionis made in the complex domain, to include gain and phase corrections.Interpolation may be implemented either in the time domain or thefrequency domain.

For example, interpolation may be implemented using a low pass filter ora FIR or convolver.

Multipath may interfere with reception of wideband signals. It may causeseveral replicas of a signal to be received, each possibly having adifferent time delay, amplitude and polarity. These signals may resultin interference between adjacent transmitted frames.

A method and system for addressing the multipath problem may includeprocessing in the frequency domain. Thus, the pilots spectrum isextracted using FFT for example. Multipath may cause undesired changesin the amplitude and phase in the pilots, which are correlated from.onespectral line to the other. These changes are responsive to the timedelay in each multipath signal.

Using signal processing applied to the spectral picture (the pilotsrepresentation in the frequency domain), each pilot signal can bereconstructed. The changes in the pilots are indicative of the multipatheffects in the channel. The information thus derived may be used tocorrect for multipath. Thus, the interference because of multipath isreduced.

Moreover, multipath signals may be added to the main path signal, toactually increase the signal power to improve the signal to noise ratio.

Multipath attenuation or cancellation may be achieved using the measuredcharacteristics of the channel. Multipath can be corrected for by usingan equalizer or transversal filter. The parameters for the equalizer arederived from the measured channel characteristics. For each detectedmultipath, the filter will generate a correcting signal of the propertime delay, amplitude and polarity.

The equalizer parameters may be computed in the frequency domain,followed with an IFFT. These parameters may be applied to a transversalfilter.

The above system and method may be advantageously used in the physicallayer specification proposed as BRAN-HA/PHY, for example.

Superior performance may be achieved at lower phase noise.

Further objects, advantages and other features of the present inventionwill become obvious to those skilled in the art upon reading thedisclosure set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the spectrum of an OFDM signal, with pilots and data.

FIG. 2 illustrates the phase of the pilots versus frequency.

FIG. 3 details the block diagram of a system for implementing ASC andAFC.

FIG. 4A illustrates the phase distortion of pilots in a communicationmultipath channel, and FIG. 4B illustrates the amplitude distortion ofthe pilots.

FIG. 5 details a block diagram of a system for correcting the phase andamplitude distortion of signals in a communication channel.

FIG. 6 details a block diagram of a system for correcting the multipathdistortion of signals using a LPF.

FIG. 7 illustrates the multipath effect on the pilots in the timedomain.

FIG. 8 details a block diagram of a system for correcting the multipathdistortion of signals using means for pilots analysis.

FIG. 9 details a block diagram of a decision feedback equalizer system.

FIG. 10 illustrates a dual loop system for implementing AutomaticFrequency Control (AFC).

FIG. 11 illustrates a conceptual block diagram of the DownstreamEncoding and Modulation subsystem.

FIG. 12 illustrates a conceptual block diagram of the DownstreamDemodulation and Decoding subsystem.

FIG. 13 illustrates a conceptual block diagram of the Upstream Encodingand Modulation subsystem.

FIG. 14 illustrates a conceptual block diagram of the UpstreamDemodulation and Decoding subsystem.

FIG. 15 details the Crest Factor versus Roll-Off Factor for SingleCarrier.

FIG. 16 details the BER/SNR for different Crest Factor values, asachieved by clipping for a DVB-T 16 QAM OFDM Symbol.

FIG. 17 details the BER/SNR for different Crest Factor achieved byclipping for an Upstream 16 QAM OFDM Symbol.

FIG. 18 illustrates Out-Of-Band Spectrum mask for a 8 MHz DVB-Ttransmission

FIG. 19 illustrates the influence of linear Group-Delay in SingleCarrier system

FIG. 20 illustrates the BER/SNR of the OFDM and S.C. systems fordifferent Phase Variance (P.V.) values.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described byway of example and with reference to the accompanying drawings.

FIG. 1 illustrates the spectrum of an OFDM signal, including pilots anddata in the complex frequency domain, with amplitude axis (I) 110,amplitude axis (Q) 111 and frequency axis 12. The spectrum includes thespectrum of data, for example 131, 132, 133 and the pilots 141, 142,143.

It is assumed that the transmitted signal includes pilots of equalamplitude and being in phase. Furthermore, the pilots are equidistant inthe frequency domain. These properties are used in the presentinvention, as detailed below. The properties of the pilots are measuredand deviations from the transmitted signal are indicative of distortionsin the communication channel. The measured distortion are used tocompute the correction parameters for the channel.

FIG. 2 illustrates a possible distortion in the phase of the pilotsversus frequency.

The graph indicates an example of phase shift in the frequency domain,with transmit phase axis 151, receive phase axis 152 and frequency axis12. The transmit pilots 151 are all in phase. A time difference maycause a phase shift in the received pilots 152, as illustrated with thephase of the pilots 141, 142, 143. Such a linear change in the phase ofpilots may be caused by a time error in sampling in the receiver. Theslope of the graph is indicative of the time error.

This may be used in a receiver to correct for synchronization errors.

FIG. 3 details the block diagram of a system for implementing ASC andAFC. The intermediate frequency (IF) input channel 211 is transferred toa couple of mixers 21 for quadrature coherent detection. A delay unit(90 degrees) 22 is used to generate the quadrature reference from alocal oscillator (LO) 23.

The LO 23 may be implemented, for example, using a voltage controlledoscillator (VCO). The detected signals (I,Q) are processed in a pair oflow pass filters (LPF) 24 and are converted to digital words in analogto digital converters (ADC) 25.

The input wideband signal 211 (time domain), after being transformedinto digital form, is applied to an FFT processor 3. The FFT processorgenerates the transformed signal 63 (frequency domain).

Signal 63 may include data spectrum and pilots, as illustrated in FIGS.1 or 2. A pilots extraction unit 27 extracts the pilots from the signal63.

The system further includes a Phase Lock Loop (PLL) 28, having a firstinput 281 from the LO 23 and a second input 282, and two outputs 283,284.

The ASC unit 4 detects the slope of the phase of pilots as illustratedin FIG. 2 and computes therefrom the synchronization error.

A corrected timing signal is applied to a numerically controlledoscillator (NCO) 26. NCO 26 generates the clock for the ADC 25. Thus,the timing of the sampling of the analog signals is adjusted responsiveto the measured timing error. This will correct the timing orsynchronization error in the receiver.

Various embodiments of the invention may be implemented. For example,sample and hold means (not shown) before the ADC 25 may be used tocorrect for synchronization errors.

Thus, automatic synchronization control is achieved, wherein ASC unit 4measures, in real time, the synchronization error and closes a loop tocorrect it. The synchronization may have to change during acommunications session. The above loop will change the synchronizationas required, to achieve a system that is adaptive to changing channelconditions.

The ASC is performed automatically and without interfering with theactual communications—no additional synchronization signals are addedand no other changes are required in the transmitted signals.

Thus, a possible error in the time synchronization between signals willbe corrected. The reduction or elimination of time synchronizationerrors will keep the signals in the various channels orthogonal to eachother, as they should be.

This may reduce or eliminate a cause of interference between channels.

Thus, the system will adapt to varying channel characteristics in realtime, to achieve improved communications.

Another problem is a frequency error between the transmitted signal andthe receiver. A frequency error generates a time-varying phase error, toresult in a rotation of the pilot signals of FIG. 2 . Thus, for afrequency error the slope of the pilots will continuously change at aspecific rate.

The system may detect such a change in the slope of pilots phase and maycompute therefrom the frequency error in the receiver. This function isimplemented in the AFC unit 5. As a frequency error is detected, AFCunit 5 will issue a correction signal to VCO 23.

Thus, automatic frequency control is achieved, wherein AFC unit 5measures, in real time, the frequency error and closes a loop to correctit. The received frequency may change during a communications session.The above loop will tune the VCO 23 as required, to achieve a systemthat is adaptive to changing channel conditions.

Real-time means are used for AFC. The frequency error is evaluated basedon the rate of rotation of the pilot signals. A correction signal isgenerated accordingly, to correctly tune the receiver to the receivedsignal.

The AFC is performed automatically and without interfering with theactual communications—no additional synchronization signals are addedand no other changes are required in the transmitted signals.

The above AFC and ASC systems and methods may be useful in widebandsignals like DVB-T, for example, where there are a large number of pilotsignals available.

FIG. 4A illustrates the phase distortion of pilots in a communicationchannel. Whereas FIG. 2 illustrated a phase distortion due to a timingdelay only, an actual channel may cause a more complex distortion, wherethe phase differences between pilots does not change in a linearfashion.

Moreover, the amplitude of the pilots may change as well. That is, foreach frequency the channel introduces a distortion characterized by anamplitude and phase change in the signal. This channel effect causes adistortion in the transmitted signal and may reduce the performance ofthe communication system.

This channel effect is illustrated in the frequency domain, with phaseaxis 15 and frequency axis 12, with pilots 141, 142, 143 each possiblyhaving a different phase.

The other distortion effect is shown in FIG. 4B, that illustrates theamplitude distortion of the pilots, with amplitude axis 11 and frequencyaxis 12 and pilots 141, 142, 143 of possibly a different amplitude each.

Method for Channel Distortion Correction

The following method may be used to correct for phase and amplitudedistortion in the channel:

-   -   A. measuring the phase of each pilot in a receiver. Measuring        the amplitude of each pilot as well.    -   B. computing a correction factor for each pilot, to bring all        the pilots in phase and to an equal amplitude. The correction        factors have a phase shift component and a gain component.    -   C. applying the correction factors to the received signals.        Between each two adjacent pilots, the correction factor may be        the average of the factors for these two pilots. Alternately,        separate correction factors may be computed for each frequency        using an interpolation method. This may allow to correct each        frequency (or each output of the FFT) with its individually        computed, corresponding correction factor.    -   D. repeating steps A-C all the time, to measure the channel        characteristics in real time and to correct in real time for        changing channel properties.    -   End of method.

Preferably, the above method is implemented after achieving goodfrequency lock and synchronization in the receiver. Then, phase rotationor linear phase change effects are removed and only remains thedistortion caused by the channel to correct.

FIG. 5 details a block diagram of a system for correcting the phase andamplitude distortion of signals in a communication channel.

This system may be used to implement the method detailed above forcorrection of the phase and amplitude distortion in the channel.

A receiver 2 may receive and detect a signal, that is transferred to theFFT processor 3 for computing the spectrum of the signal. The signal inthe frequency domain is transferred to a pilots extraction and analysisunit 71.

The unit 71 includes means for:

-   -   A. extracting the pilots from the received signals    -   B. analyzing the pilots to detect distortion in phase or        amplitude, as detailed above. These distortions are indicative        of the distortion in the communication channel.    -   C. computing the complex correction coefficients for the various        frequencies in the signal, using information derived from pilots        in step (B) above. A possible method may use interpolation.        Averaging of adjacent pilots or other methods may be used as        well.    -   D. applying the correction coefficients, as correction signals        64 (phase and amplitude), to the signal correction unit 72.

The transformed signal 63 (frequency domain) is transferred to unit 72,where the correction coefficients are applied to correct it. Thisresults in the corrected signal 65 (frequency domain) out of unit 72.

The above system and method may be used to implement a channel sounder.Using means for analyzing the received pilot signals, a signal processorcan characterize the communication channel.

The phase and amplitude of the pilots is measured to evaluate thechannel distortion at different frequencies. The results are used tocorrect the received signal accordingly. Interpolation may be used tocorrect for phase and amplitude of received signals between any twoadjacent pilots.

This system and method corrects the distortion of the signal frequencyspectrum, to improve the receiver performance.

FIG. 6 details a block diagram of a system for correcting the multipathdistortion of signals using a Low Pass Filter LPF.

The system includes a receiver 2 for a received signal. The inputwideband signal (time domain) from receiver 2 is transferred to an FFTprocessor 3, that generates a transformed signal in the frequencydomain. This signal is transferred to a pilots extraction and analysisunit 71, that extracts the pilots from the received signal. A Low PassFilter (LPF) 73 is used to measure the multipath, applying a time-domainprocessing to the pilots spectrum that is presented to the LPF as atime-varying signal. Multipath causes changes in the pilots, that aredetected in the LPF.

The resulting multipath information is applied to a channel equalizer74. The channel equalizer 74 also receives the received signal (infrequency domain) from the FFT processor 3. Unit 74 then corrects thereceived signal for multipath. The corrected signal 65 (frequencydomain) is the output of the system.

The above system may be used to correct for multipath, that mayinterfere with the reception of wideband signals. It may cause severalreplicas of a signal to be received, each possibly having a differenttime delay, amplitude and polarity. These signals may result ininterference between adjacent transmitted frames.

The LPF as detailed is one possible embodiment of means for timefiltering in the frequency domain. The LPF is applied to the spectralpicture (the pilots representation in the frequency domain), so thateach pilot signal can be reconstructed. Multipath signals are added tothe main path signal, to actually increase the signal power to improvethe signal to noise ratio. Furthermore, the interference because ofmultipath is reduced.

FIG. 7 illustrates the multipath effect on the pilots in the timedomain, with amplitude axis 11 and time axis 16. The signal illustratedis one example of multipath. The pilots are extracted from the signaland combined in the time domain. A pulse train in the frequency domainwill result in a pulse in the time domain, this is the pilots pulse 17.

If there is multipath, it will result in a pulse with a specific delay,according to the time delay of the multipath channel in thecommunication path. Thus, for example, the channel may have a firstmultipath pulse 171 and second multipath pulse 172, having a time delay161 and 162, respectively.

FIG. 8 details a block diagram of a system for correcting the multipathdistortion of signals using means for pilots analysis.

The system may use the above detailed multipath effect, as detailed withreference to FIG. 7.

An FFT processor 3 computes the spectrum of the received signals, thatis transferred to unit 71. The pilots extraction and analysis unit 71extracts only the pilots in the received signal. The pilots dataundergoes an inverse FFT in IFFT unit 75. The output 751 of unit 75 mayhave the shape illustrated in FIG. 7, that is each multipath pathresults in a pulse with a characteristic amplitude, time delay andpolarity. Output 751 comprises the channel sounder output of the system.

The information regarding each multipath is applied to an equalizercoefficients calculation unit 77.

Unit 77 computes the coefficients to be used in channel equalizer unit76, responsive to the measured channel information from the channelsounder. The computed coefficients are transferred to unit 76.

The unit 76 operates in the time domain to add or subtract each signalfrom multipath, to result in a corrected signal 66 (time domain).

Thus, multipath attenuation or cancellation is achieved using themeasured characteristics of the channel.

Multipath can be corrected by using an equalizer or transversal filter.For each detected multipath, the filter will generate a correctingsignal of the proper time delay, amplitude and polarity.

As multipath is corrected, two benefits may be achieved: a signal withno multipath or reduced multipath may result in improved communications;and, since now the multipath signal is added in phase, it may actuallyincrease the power of the received signal, to improve the signal tonoise ratio in the system.

FIG. 9 details a block diagram of a decision feedback equalizer system(DFE).

The system implements a multi-stage equalization and error correctionmethod to be detailed below.

An input (baseband) 960 is connected to a recording unit 961. Thisallows the same frame to be played several times into the processingsystem. This allows for a simpler, lower cost implementation. Otherwise,separate units may be used for the various processing stages, and theunit 961 may not be required in that case.

A combiner 962 combines the input signal from unit 961 with feedbacksignals from a processor, that may be implemented with FIR 975 andcombiner 976.

A FIR 963 filters the input signals, together with a FIR combiner/bypassunit 964.

An equalizer coefficients calculation unit 969 provides the coefficientsfor the FIR. Alternately, only the middle tap of the FIR is output tothe FFT 965. To this effect, unit 969 sets all the FIR coefficients tozero, except the middle tap, that is set to 1 or other nonzero value.

After the FFT in unit 965, the signal is transferred to pilot extractionunit 967. This is followed by IFFT 968 and the equalizer coefficientscalculation unit 969, based on the pilots values in the time domain.

A switch 971 allows to transfer the equalized received signal to errordetection and correction unit 972 (EDC). The output 973 is the dataoutput of the system, after equalization and error detection andcorrection.

A transmit signal synthesizer 974 is used to generate a replica of thereceived signal with the estimated multipath, in combination with FIR975 and combiner 976.

The resulting signal is applied to combiner 962 to remove multipath tofurther enhance the received signal.

Equalization and Error Correction Method

The system detailed in FIG. 9 may implement a decision feedbackequalizer method comprising the following steps:

-   -   A. record a frame of received data    -   B. received data passes through an equalizer (FIR) that is set        to bypass mode, that is all the FIR coefficients are set to        zero, except the middle tap, that is set to 1 or other nonzero        value. This will not filter the signal, however the delay of the        FIR is taken into account.    -   C. perform an FFT of the received frame    -   D. pilots extraction    -   E. IFFT    -   F. FIR coefficients calculation and application to the FIR.        Subsequent frames may be used to update the coefficients in a        pipeline fashion. Thus, in future frames the step (B) will use        coefficients computed for the previous frame rather than zero        value coefficients.    -   G. the recorded frame is again applied to the system, however        this time the equalizer (FIR) corrects the input data according        to the measured coefficients.    -   H. error detection and correction    -   I. a replica of the transmitted signal is synthesized, based on        the corrected input signal. The synthesized signal contains the        measured multipath signals, that are generated in a FIR and        combiner.    -   J. the recorded frame is again applied to the system, however        this time the replica of the multipath is subtracted from the        input signal.    -   K. error detection and correction    -   L. output data.    -   End of method.

A possible problem in wireless is a frequency error between thetransmitted signal and the receiver.

The frequency resulting from the AFC loop is used as a clock for thereceiver and subsequently for the transmitter. A frequency error maystem from two possible causes:

-   -   A. an undesired difference between the receiver LO (local        oscillator) and the transmit LO.    -   B. a frequency Doppler shift because of the movement of the        mobile subscriber.

This effect, together with means for its correction using a dual loopAFC, are detailed with reference to FIG. 10.

FIG. 10 illustrates a dual loop system for implementing AutomaticFrequency Control (AFC).

The system includes an inner local loop in the subscriber unit, and anouter loop implemented with components both in the subscriber unit andthe base station.

The inner loop includes an AFC loop 822 connected to a subscriberreceiver 821 for locking the frequency of receiver 821 to the frequencyof the signal received from the base station. For example, unit 822 maylock the local oscillator to a pilot signal received from the basestation. Accordingly, unit 822 generates a receiver clock 8221 for thereceiver 821. Unit 822 also generates a transmitter clock 8222 that istransferred to the means for generating the transmit frequency. In theexample as illustrated, the embodiment of these means is the DDS Tx 826.

The transmit frequency out of unit 826 is used in the Tx subscriber 827,that is the transmitter of the subscriber unit, for transmission to thebase station.

This loop solves the problem of tuning the mobile receiver to the basestation transmissions. The subscriber frequency may be in error,however, for various reasons. For example, movement of the subscriberunit may result in a Doppler frequency shift of the signal received fromthe base station. The receiver will lock to the shifted frequency.

The signal received at the base station will have double that frequencyshift, because of the relative movement between base and mobile station.

As various subscriber units will transmit with a frequency error, thereceiver in the base may have difficulty in effectively separating thesereceptions.

To solve these frequency errors, a second (outer) loop is added, whereinthe base stations measures the frequency deviations of each subscriberand issues instructions to each subscriber to correct its transmitfrequency.

The outer loop is implemented, in the example as illustrated in FIG. 10,as follows: The BS Rx 812 (base station receiver) receives transmissionsfrom mobile subscribers.

Frequency offset unit 813 measures the frequency error in the receivedsignal, that is the difference between the actual received frequency andthe precise frequency that was allocated to that subscriber. The resultsof the measurement are transferred to a frequency correction (Up/Down)unit 814. Unit 814 generates frequency correction messages 815 that aretransmitted through the BS Tx 811 (base station transmitter) to themobile subscriber.

In the mobile unit, these messages are received in receiver 821 and aretransferred to the information extraction unit 823. The decoded messagesare transferred to the AFC loop closing unit 824, that controls theinstruction from base application unit 825.

The reconstructed frequency control signals (frequency correctionUp/down instructions) are transferred to the DDS 826.

The DDS 826 includes means for performing a frequency shift according tothe instructions received from unit 825.

Thus, the frequency at the output of DDS 826 is derived from thefrequency of the received signal, corrected according to instructionsfrom the base stations.

The inner, local frequency control loop sets the frequency according tothat of the received signal.

The outer frequency control loop corrects the above frequency settingaccording to instructions from the base station.

The DDS 826 actually forms the transmitter local oscillator. Its outputis transferred to the transmitter 827.

The above system and method may be advantageously used in the physicallayer specification proposed as BRAN-HA/PHY, for example. Following is adetailed description of this embodiment of the invention and itsestimated performance.

It uses an OFDMA access method for the access method for BRAN-HA /PHYFollowing is a description of this embodiment of invention.

1. Overview

Following is a general description of a physical layer specificationproposed as the BRAN- HA/PHY. In order to leverage existing technologyand reduce costs this proposal uses many of the ETSI Digital VideoBroadcasting (DVB) standard for terrestrial broadcasting in thedownstream channel (Base Station to Subscriber Unit). In addition, thisproposal includes physical elements and implementation aspects thatspecifically address the challenges to operating reliably in the 20-60GHz band.

2. Duplexing Technique

The proposed physical layer is based on Frequency Division Duplexing(FDD), which provides a separate frequency assignment for the upstreamand down stream channels. We can also use a modification of the OFDMmodulation parameters in order to operate the system in Time DivisionDuplexing (TDD) or in Half Frequency Division Duplexing (H-FDD).

3. Multiple Access Method

The proposed upstream physical layer is based on the use of acombination of Time Division Multiple Access (TDMA) and OrthogonalFrequency Division Access (OFDMA). In particular, the upstream isdivided into a number of “time slots” as defined by the MAC layer. Eachtime slot (sized to duration of one OFDM symbol) is then divided in thefrequency domain into groups of sub-carriers referred to as subchannels.The MAC layer controls the assignment of subchannels and time slots (bybandwidth on demand and Data Rate on demand). This initial proposalfocuses on the efficient transport of ATM cells and IP packets in theupstream and down stream channels.

4. Downstream Transport Stream and Physical Layer

The downstream physical layer uses aspects of the well-proven DVB-Tphysical layer. This standard uses the OFDM as its modulation technique.This standard is based on the transmission of packetized digital videocorresponding to MPEG-2. In particular a transmission convergence layercan be designed to efficiently transport ATM cells and IP packets(although any frame structure can be used, the MPEG-2 is widely usedtoday). An OFDM symbol will be divided (in the frequency domain) intogroups. The first group is a group, which will be dedicated for thebroadcast of MPEG-2 transport and can be used in a SFN as thebroadcasting area.

The MAC layer for fast feedback or response will use another group, thelast group will be allocated for dedicated channels and could carrydifferent information in a SFN configuration. We shall indicate that thebroadcasting subcarriers group shall vary as needed, if there is no needfor any broadcasting all of its subcarriers group shall be used by thededicated channels. The encoding and decoding functions for thedifferent group types are summarized in the next block diagram, thefunctions for the MPEG-2 data stream and for the dedicated channels areadopted from the DVB-T standard. (FIG. 1).

FIG. 11 illustrates a conceptual block diagram of the DownstreamEncoding and Modulation subsystem. The subsystem may be used for severalchannels, for example one for broadcasting MPEG-2 850, another fordedicated MPEG-2 851 , and one for MAC messages 852 . The processing ineach channel may include a randomization unit 830, an RS coder (204,188)831, a convolutional interleaver 832, convolutional encoding andpuncturing unit 833, bit interleaver 834 and a symbol mapper 835.

The plurality of channels as illustrated (for example one forbroadcasting MPEG-2 850, another for dedicated MPEG-2 851 , and one forMAC messages 852) are then processed in the IFFT unit 838. The resultedsignal is transmitted over transmission channel 839.

For the MAC messages 852, the processing preferably includes an RS coder(26,20) 836 and a small convolutional interleaver 837.

FIG. 12 illustrates a conceptual block diagram of the DownstreamDemodulation and Decoding subsystem. The signals input over thereception channel 849 are processed in a FFT unit 848. The separateresulting data channels are each processed in a symbol demapper 845, bitdeinterleaver 844, convolutional decoding unit 843, convolutionalinterleaver 842, RS decoder 841 and randomization unit 840.

The subsystem is devised to output the data in several channels as sent,for example one for broadcasting MPEG-2 853, another for dedicatedMPEG-2 854, and one for MAC messages 855. Some of the channels mayinclude a small convolutional interleaver 847.

The transport stream is, therefore, very robust and can be changed as afunction of the protection against fading, noise and distance thatshould be reached.

Different modulation schemes QPSK, 16 QAM, 64 QAM and differentpuncturing rates ½, ⅔, ¾, ⅚, ⅞ enables an optimization of the Downstreambit rate and protection. Moreover at condition of LOS the guard intervalneeded to mitigate the multipath affects is very small, therefore a useof a small guard interval increases the channel capacity. The Guardintervals supported should then be 1/256, 1/128, 1/64 (see calculationsection). For a SFN deployment a larger Guard Interval of 1/32, 1/16, ⅛can be introduced.

5. Upstream Physical Layer

The upstream physical layer is also based upon OFDM modulation, thenumber of subchannels allocated to a specific user and the timing theywill be transmitted in a specified time frame are controlled by the MAClayer. Since the upstream is TDMA/OFDMA based the channel can be modeledas a continuos sequence of “time slots” and each time slot can bemodeled as a group of subchannels that are allocated to differentSubscriber Units by Bandwidth On Demand. By using this technique, QoSrequirements and bandwidth requirements can be managed. The recommendedcoding and modulation of upstream packets are summarized in the blockdiagram shown in FIG. 13. As shown in the diagram such a coding schemeis used in order to support a large granularity for the bandwidth ondemand requirements.

FIG. 13 illustrates a conceptual block diagram of the Upstream Encodingand Modulation subsystem. The figure illustrates a reverse channeltransmit, for example for MPEG-2 850. The signal processing includes ade-randomization unit 860, variable RS coder 861, small convolutionalinterleaver 862, convolutional encoding and puncturing unit 863, symbolmapper by allocation 865 and IFFT unit 868.

The resulting signals are transmitted over the transmission channel 869.

FIG. 14 illustrates a conceptual block diagram of the UpstreamDemodulation and Decoding subsystem.

The figure illustrates an embodiment of signal processing of signalsreceived over the reception channel 879.

The signal processing includes a FFT unit 878. From the outputs of unit878, a plurality of channels may be formed, according to the initialcarrier allocation at transmission.

In each channel, the signals are processed in a symbol de-mapper bysub-channel allocation 875.

Further means for signal processing include a convolutional decodingunit 873, small convolutional deinterleaver 872, variable RS decoder 871and a de-randomization unit 870.

The resulting signal is transferred to output the data in MPEG-2streaming 854 per user.

Every subchannel may consist of several carriers (see calculationspart), most are used for data transmission and the rest are used forpilots transmission.

6. Physical Layer Properties

The next section deals with different aspects of the physical layerimplementation.

6.1 Synchronization Technique/Timing Control

In order to avoid highly accurate frequency source (e.g., OCXO) at theSubscriber Unit and satisfy timing requirements for telephony or otherCBR applications (Tl/El), it is highly efficient to derive theSubscriber Unit's clocks from the Downstream transmission. This can beachieved by using the Pilots carriers transmitted by the Base Station,these Pilots can also be used in order to Synchronize onto theDownstream transmission and achieve clock extraction. Accurate upstreamtime slot synchronization shall be supported through a rangingcalibration procedure defined by the MAC layer using the pilotstransmitted by each Subscriber Unit.

Moreover, the Base Station copes with users transmission not arrivingfully synchronized, and relieving the demand for users synchronization.

6.2 Frequency Control

The clock extracted from the Downstream (as explained before) is used asthe reference clock of the Subscriber unit, in particular to produce theRF frequency for the transmission and to adopt this clock as theSubscriber Unit Base Band clock. Locking on the Downstream transmissionfrequency shall allow an accurate Upstream RF transmission frequency tobe produced, that ensures that all Subscriber Units transmitting shallreach the Base Station Orthogonal, keeping the OFDM properties.

6.3 Power Control

In order to perform a Upstream power control the Base Station shall usea calibration and a periodic adjustment procedures. The adjustmentvalues shall be sent to a Specific Subscriber Unit via the MAC layer.The Base station shall extract the adjustment values by monitoring thepower on the carriers that were allocated to the specified user on thespecified OFDM symbol. Controlling the power of the Downstream dedicatedchannels will perform another power control mechanism. The specifiedSubscriber Unit MAC shall send adjustment values to the base stationcorrecting the power transmitted on the dedicated channel, and adjustingit to the demands of a certain SNR. This procedure will enable anoptimized use of the base station Power Amplifier.

6.4 Crest Factor

Much research has been done on the crest factor of OFDM modulation.

The maximum crest factor is derived using 10*log(N), where N is thenumber of carriers used in the OFDM symbol. Taking into considerationthat in our suggested system we use a 2048 carriers FFT/IFFT which isvery similar to the “2k” mode of the DVB-T we shall introduce somemeasurements done on the DVB-T.

In the DVB-T, 1705 carriers are used for carriers transmission, a crestfactor of 32.3 dB would be expected but in fact only 9-9.5 dB crestfactor (with peaks of 10.5 dB) is actually measured in any modulationusing QPSK, 16 QAM and 64 QAM. These results are achieved by therandomization of the data sent on the carriers. In comparison to asingle carrier modulation using 64 QAM and a roll-off factor of0.25-0.35 we get a crest factor of 8.8-7.8 dB, for 16 QAM we get a 1.4dB reduction, resulting in 7.4-6.4 dB see FIG. 15.

FIG. 15 illustrates the Crest Factor versus Roll-Off Factor for SingleCarrier.

In order to further reduce and stabilize the crest factor we can clipthe signal in order to achieve a desired crest factor. The next graphplots BER/SNR for different crest factor limitations for a DVB-T 16 QAMOFDM symbol, see FIG. 16.

FIG. 16 illustrates the BER/SNR for different Crest Factor values, asachieved by clipping for a DVB-T 16 QAM OFDM Symbol

As we can notice, for a 1-1.5 dB clipping we get no performancedegradation, for a 2-2.5 dB clipping we get only about 0.5 dBdegradation. For a 64 QAM modulation a degradation of 0.5 dB could beachieved when clipping 1.1-1.6 dB, therefore achieving a steady crestfactor of 7.8 dB. By using more sophisticated methods, more reductioncan be achieved.

For the Upstream where a reduced number of carriers are used (takinginto consideration that all useful carriers are divided into 16subchannels), the crest factor achieved is about 7-7.5 dB for QPSK, 16QAM and 64 QAM all modulations (with peaks of 9.5 dB).

Taking the same method as before, for a 16 QAM modulation clipping thepower in such a way that the crest factor is 6.5 dB will introduce onlyabout 0.2-0.4 dB degradation, see FIG. 17.

By using more sophisticated methods, more reduction can be achieved.

FIG. 17 illustrates BER/SNR for different Crest Factor achieved byclipping for an Upstream 16 QAM OFDM Symbol

6.5 Spectrum Properties

The spectrum properties of an OFDM modulation are derived from theFFT/IFFT properties, although there is a natural decay in theOut-Of-Band frequency domain a much tighter spectrum is achieved byusing additional measures. For an example the Out-Of-Band spectrum maskfor a 8 MHz DVB-T transmission is shown in the FIG. 18. FIG. 18illustrates Out-Of-Band Spectrum mask for a 8MHz DVB-T transmission.

Comparing OFDM to a Single Carrier where using a roll-off factor of0.25-0.35, it can be seen that OFDM modulation achieves much moreefficient spectrum properties with no degradation in the performance,whereas in the Single Carrier there is a degradation of 0.5-1.5 dB

6.6 Power Amplifier Efficiency

From sections 6.3-6.5, we notice that for high modulation scheme, thecrest factor of an OFDM transmission can be achieved to be even lowerthan for single carrier transmission. Furthermore, considering thespectrum efficiency of the OFDM modulation, we can derive that the poweramplifier usage for an OFDM transmission is very high, and a powercontrol mechanism allows the better usage of the Power Amplifier. Inparticular, these conclusions are enhanced for an Uplink transmission,while for a Single Carrier transmission the same power efficiency isachieved.

For an OFDM transmission, where the user is allocated a subchannel, thetotal power transmitted is divided between less carriers, to achieve anadditional power gain of 12 dB (for a case were the symbol is dividedfor 16 users).

6.7 Timing Sensitivity

In an OFDM modulation, there is no timing sensitivity within the sampletime and simple phase and channel estimators correct inaccuracies.Furthermore the Guard Interval of the transmissions insures immunity inthe face of multipath or unsynchronized reception of OFDM transmissionfrom several sources. In particular this fact enables the creation ofSFN on the DownLink, and of a very relaxed timing synchronizationdemands of Subscriber Units in the Uplink.

6.8 Frequency Sensitivity

OFDM symbol demodulation is sensitive to frequency inaccuracies. Thissensitivity is solved by accurate AFC loops using DDS. Using the aboveapproach all Subscriber Units lock on the Base Station frequency asexplained in 6.2. In doing so they ensure that their own transmission iskept orthogonal to other Subscribers, and the total OFDM symbol shallremain orthogonal.

6.9 Equalizations

While in Single Carrier equalizers are a must, and the transmission of atraining sequence (and the lost of data rate) is needed, in an OFDMsystem time sensitivity is relaxed and a channel estimator is the onlything needed in order to fix the timing demands and channel imparities.

6.10 Group Delay

The same channel estimators mentioned in 6.7-6.9 can compensate groupDelay caused by filters. The Group Delay introduced is treated like achannel imparity. Single Carrier systems are very much influenced byGroup Delay as Shown in FIG. 19. In our System, it is expected to be inthe 0.15-0.2 (see calculation and assuming a group delay of 10 nsec).

In our System, it is expected to be in the 0.15-0.2 Tm/T (seecalculation). FIG. 19 illustrates the influence of linear Group-Delay inSingle Carrier system.

6.11 Burst Efficiency

Upstream bursts of Subscriber User are very efficient because of a lowoverhead. Subscriber Unit that has been allocated to one subchannel hasonly 14% (16 of 112 carriers) of the carriers dedicated to pilots (theseare used for all receiver demands for time, power and frequency control,and are also used for channel estimation). If user is allocated moresubchannels there is no need for further increase of pilots number, sofor 2 subchannel efficiency shall rise and the overhead decreases to 7%(16 of 224 carriers), if all band is given to the user the overheadshall be less than 1% .

6.12 Sectorization, Cross Polarization and Diversity

Sectorization, Cross Polarization and Diversity can be used in an OFDMAsystem as well, and may give many advantages.

7. Comparison between OFDMA and Single Carrier TDMA

The following table is a rough comparison between OFDMA and a SingleCarrier System using TDD, numbers were derived from experience,simulations and articles.

Table

8. Calculations

-   The next calculations are for the Downlink/Uplink transmissions.-   Bandwidth=28 MHz-   OFDM Carriers=2048-   Carriers in use=1792-   Sample Rate=28 MHz * (2048/1792)=32 MHz-   Carriers Distance=Bandwidth/Carriers in use=15625 Hz-   Guard interval 1/128=16 samples=500 nsec-   Frame duration=(2048+16)/32 MHz=64.5 usec    8.1 Downlink-   Pilot Carriers per OFDM symbol=80 carriers-   Data carriers in use=1792−80=1712-   Symbol rate=1712 carriers/Frame Duration=26.543 Msps-   Total throuput (QPSK) before ECC=53.085 Mbps-   Total throuput (16 QAM) before ECC=106.17 Mbps-   Total throuput (64 QAM) before ECC=159.26 Mbps    8.2 Uplink-   Number of Carriers used for Uplink contention=64-   Number of Subchannels per OFDM frame=16 Subchannels-   Number of carriers per on Sub channel allocation=108 carriers-   Pilot Carriers per Subscriber Unit=16 carriers-   Data carriers assuming n Subchannel for a specified Subscriber Unit    (n ranging from 1 to 16)=108*n−16-   Data carriers assuming 1 Subchannel for a specified Subscriber    Unit=108−16=92 carriers-   Data carriers assuming 16 Subchannel for a specified Subscriber    Unit=1792−64−16=1712 carriers-   Symbol rate assuming best subchannel allocation (all Subchannels per    Subscriber unit)=(1792−64−16) carriers/Frame Duration=26.543 Msps-   Symbol rate assuming worst subchannel allocation (one per Subscriber    unit)=(1792−64−16*16) carriers/Frame Duration=22.822 Msps-   Symbol rate per subchannel (Worst allocation)=1.4264 Msps-   Total throuput (QPSK) before ECC, worst allocation=45.643 Mbps-   Total throuput (16 QAM) before ECC, worst allocation=91.287 Mbps-   Total throuput (64 QAM) before ECC, worst allocation=136.93 Mbps-   TDMA frame length=16 OFDM symbols-   TDMA frame duration=16 * 64.5 usec=1.032 msec    9. Phase Noise Simulations

The following analysis deals with the influence of phase noise on OFDMand Single Carrier Systems.

In order to check the phase noise influence a simulation was written inMATLAB, using a model suggested in prior art.

The model simulates the phase noise by using a white Gaussian processfiltered with a single pole low pass filter, the rational for using thismodel is a typical behavior of phased-locked microwave oscillators.

The spectrum for the phase noise simulation has a Phase Variance of −26dB.

Using this Phase Noise model we tested an OFDM and a Single Carrier(S.C) system for their BER/SNR performance with different Phase Variance(P.V) values. The OFDM system used is more precisely described in priorart. We will just indicate that the system uses a 28 MHz bandwidth andhas 2048 carriers, the system works with a 32 MHz clock. The S.C. systemused has the same bandwidth and works with a 28 MHz clock, no pulseshaping has been applied. Both systems were tested for a 16 QAMmodulation.

FIG. 20 illustrates the BER/SNR of the OFDM and S.C. systems fordifferent Phase Variance (P.V.) values.

10. Conclusions

It will be noticed that the difference between the systems is minor andis in the favor of the OFDM system. For a synthesizer that has a betterPhase Variance than −40 dB, no performance degradation occurs. For asynthesizer with a Phase Variance of −26 dB a degradation of 0.5-2 dBoccurs.

Such a synthesizer has a phase noise of about −80 dBc at 1 KHz and −90dBc at 10 KHz.

These conclusions are different from some results presented in priorart. However, the results from the simulation are consistent to thoseachieved in prior art as summarized in CHAYAT, May 1998.

Various modifications of the preferred embodiment are possible withoutdeparting from the scope of the present invention, and many of thesewould be obvious to people skilled in the art.

Although the invention has been described in connection with a preferredembodiment, it is to be understood that this description is not intendedto limit the invention thereto. Rather, the invention is intended tocover all modifications and/or additions to the abovementioneddescription, without departing from the spirit and scope of theinvention.

1. In an OFDM-based receiver, means for achieving time synchronizationcomprising: A. means for extracting pilot signals contained in the OFDMreceived signal; B. means for analyzing the pilot signals in thefrequency domain and for issuing a signal indicative of asynchronization error in the received signal; and C. means forcorrecting the synchronization error responsive to the signal indicativeof the synchronization error:
 2. The synchronization means according toclaim 1, wherein the means for extracting pilot signals comprise FFTmeans and signal processing means in the frequency domain.
 3. Thesynchronization means according to claim 1, wherein the means forextracting pilot signals, the means for analyzing the pilot signals andthe means for correcting the synchronization error operate continuouslyin real time to keep the OFDM receiver synchronized.
 4. Thesynchronization means according to claim 1, wherein the means foranalyzing the pilot signals in the frequency domain include means formeasuring the rate of rotation of the pilot signals.
 5. In an OFDM-basedreceiver, automatic frequency correction means in a subscriber unitcomprising: A. an inner frequency correction loop for generating a LOfrequency related to a frequency of a received signal; and B. an outerfrequency correction loop for correcting the LO frequency according toinstructions received from a base station.
 6. The automatic frequencycorrection means according to claim 5, wherein the inner frequencycorrection loop includes means for locking to the frequency of thereceived signal.
 7. The automatic frequency correction means accordingto claim 5, wherein the outer loop includes DDS means for generating asignal at a frequency derived from that of the received signal, modifiedaccording to the instructions received from the base station.
 8. In anOFDM-based receiver, a channel sounder comprising: A. means forextracting pilot signals contained in the OFDM received signal; B. meansfor analyzing the pilot signals in the frequency domain and for issuingsignals indicative of a distortion in each pilot signal, wherein each ofsaid pilot distortion signals comprises both an amplitude and a phasecomponent; and C. means for analyzing the signals indicative of adistortion in each pilot signal and for computing therefrom correctivesignals for correcting distortions in the received signal.
 9. Thechannel sounder according to claim 8, wherein the correction of thereceived signal is performed in the complex domain, to include both gainand phase corrections.
 10. The channel sounder according to claim 8,further including means for computing an average distortion of twoadjacent pilots and for using that average to correct the informationbetween these pilots.
 11. The channel sounder according to claim 8,further including means for computing, for each frequency between twoadjacent pilots, an interpolated value of the distortion, and for usingthat interpolated value to correct the information at that frequency.12. The channel sounder according to claim 11, wherein the interpolationis performed in the time domain or the frequency domain.
 13. The channelsounder according to claim 11, wherein the interpolation is performedusing a low pass filter or a FIR or convolver.
 14. In an OFDM-basedreceiver, a multipath cancellation system comprising: A. means forextracting pilot signals contained in the OFDM received signal; B. meansfor analyzing the pilot signals in the frequency domain for generatingsignals indicative of multipath reflections; and C. equalizer means forreducing multipath, wherein the parameters of the equalizer arecontrolled by the signals indicative of multipath reflections.
 15. Themultipath cancellation system according to claim 14, wherein theequalizer means comprise a transversal filter.
 16. The multipathcancellation system according to claim 14, wherein the analyzing meanscomprise processing in the frequency domain, followed with an IFFT.